Optimal texture size and using?

[imagine]

I saw in a tutorial video a mapper creating 512x512 textures, packing them in .pk3 with Slade and using in UDB. So I did the same in a test map and found out that a 512x512 texture looks nice enough with a scale factor of ~4.



1. But what is an optimal texture size? I mean, for an average low-budget modern PC (say, with a built-in GPU), not ancient hardware. Are there some rules of thumb? I want the textures to look as hi-res as possible.

2. Should the textures be square? Should they be a power of two (128, 256, 512)?

3. As you see, I use two different textures for the locker: one has a stain, and the other doesn't. The stain now is a layer in Photoshop. How do I use layered textures correctly: a) prepare two large textures in a graphical software or b) make layers in UDM? If the latter, how do I create a layer? Is it the flat object? I read in the UDM manual about the flats, but not sure, whether they are the layers. Anyway, what is the right way of doing layered texturing?

Thank you an advance!

[axredneck]

Should they be a power of two (128, 256, 512)?

The less 1's the binary number has, the faster the computer processes it, so yes, they should.

[axredneck]

Edit:

As you see, I use two different textures for the locker: one has a stain, and the other doesn't. The stain now is a layer in Photoshop. How do I use layered textures correctly: a) prepare two large textures in a graphical software or b) make layers in UDM? If the latter, how do I create a layer? Is it the flat object? I read in the UDM manual about the flats, but not sure, whether they are the layers. Anyway, what is the right way of doing layered texturing?

I know at least three ways: midtextures, decals and patches (TEXTURES lump).
https://doomwiki.org/wiki/3D_middle_texture
https://zdoom.org/wiki/Decal
https://zdoom.org/wiki/TEXTURES
I usually prefer midtextures.